possible fix for mysterious crash in native code

platform/android/AndroidManifest.xml

@@ -1,8 +1,8 @@
 <?xml version="1.0" encoding="utf-8"?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
 	  package="com.cgogolin.penandpdf"
-	  android:versionCode="66"
-	  android:versionName="1.3.5"
+	  android:versionCode="67"
+	  android:versionName="1.3.6"
 	  android:installLocation="auto">
   <permission android:name="com.cgogolin.penandpdf.LAUNCH_PEN_AND_PDF_FILE_CHOOSER" />
   <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

platform/android/src/com/cgogolin/penandpdf/MuPDFCore.java

@@ -110,13 +110,13 @@ private native void updatePageInternal(Bitmap bitmap,
     private native void abortCookie(long cookie);
     private native boolean cookieAborted(long cookie);
 
-
-    public native void setInkThickness(float inkThickness);
-    public native void setInkColor(float r, float g, float b);
-    public native void setHighlightColor(float r, float g, float b);
-    public native void setUnderlineColor(float r, float g, float b);
-    public native void setStrikeoutColor(float r, float g, float b);
-    public native void setTextAnnotIconColor(float r, float g, float b);
+        /* making these non synchronized probably lead to a hard to debug crash in native code */
+    public synchronized native void setInkThickness(float inkThickness);
+    public synchronized native void setInkColor(float r, float g, float b);
+    public synchronized native void setHighlightColor(float r, float g, float b);
+    public synchronized native void setUnderlineColor(float r, float g, float b);
+    public synchronized native void setStrikeoutColor(float r, float g, float b);
+    public synchronized native void setTextAnnotIconColor(float r, float g, float b);
     public synchronized native int insertBlankPageBeforeInternal(int position);
 
 	public synchronized native boolean javascriptSupported();